inline_entity_form_form_alter changes to submit elements should happen as late as possible

Problem/Motivation

We have some custom modules that are relying on certain items and adding things later in #process callbacks. IEF breaks this when submit handlers are copied to the submit element in hook_form_alter.

An example in contrib where this breaks is the File Entity Inline module.

Proposed resolution

Make this run as late as possible and do actual copying of submit handlers in an after_build callback instead. This way we can make sure this is running as late as possible.

Remaining tasks

User interface changes

N/A

API changes

N/A

Data model changes

N/A